Mechanical Rail Accelerator - Non-electromagnet "rail" Gun

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Introduction: Mechanical Rail Accelerator - Non-electromagnet "rail" Gun

There's been all sorts of coil guns and rail guns being posted... but here's an interesting alternative. Not really useful, but amusing and effictive as a physics demo. The concept? Conservation of momentum

You'll need some magnets -preferably of the same size, type and somewhat strong (weak magnets do not work as effectivly). You're also going to need some ball bearings that show visible reactions in magnetic fields (aka nickel, coblat and iron/ferrous steel).

You're also going to need a ruler or some sort of straight edge. Movie on next step ;)

Step 1: Setup and Demonstration

I'm using the magnets found inside the magnetix brand of magnetic toy.

Place one magnet on one ball - then place the ball on your straight edge. Attach to the ball (but not to the magnet), one or two additional balls. You can stop here - or repeat this as many times as you want/can.

Take a different ball - and roll it towards the magnet on the first collection of magnet/balls. You should notice that once the ball is inside the magnetic field, it accelerates towards the magnet and makes contact with the magnet/ball cluster. Once contact is made, the last ball(s) separate in the same direction as the original ball.

This phenomina demonstrates the concept of conservation of momentum. If you were to repeat this demonstration without the magnets, you will have the same results with less motion.


The rest of this instructable has more information/examples for those that want to know more.

Step 2: Giant Clusters - Efficiency

This setup demonstrates what happens when the force of impact is great enough to separate more than one ball.

Setting up:
1st Cluster: Two magnets with four balls
2nd Cluster: Two magnets with five balls

Gently roll the ball into the first cluster. Depending on how much energy you give the first ball (how fast you push it), an additional ball me separate from the first cluster (see second picture). why? The momentum collected just before the initial impact overcame the force required to separate one ball and also the second ball. Ideally, you want all of this momentum going into the first ball rather than the available momentum being split between two different balls.


Going further -- increasing efficiency.

Now, remove one ball from the first cluster. But leave the second cluster alone. (See third picture) Repeat the experiment.

Notice in the fourth picture (after impact), that the first cluster remained intact but the second cluster is missing two balls! why? Because more momentum was put into the ball ejected off the first cluster (simply by removing one ball from the system), the momentum going into the second cluster was great enough to remove two balls.

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why do the balls stick together in the first place? In case someone needs to ask. This is due to the magnetic force between the balls and magnet. Even the final ball has a magnetic force on it, but that force is weaker compared to each ball closer to the magnet. To proove this - make the first cluster made up of one ball. Now, gently push another ball into it. The ball/magnet system will not separate because the force between the magnet and ball is too great.

Step 3: Ramps!

What happens when we go up a ramp?

To demonstrate a special case, I setup this demo to show what happens at a sort of "critical point."

From the first image, you can see that the second cluster is quite large -- and there are balls on either side of the magnet. We will gently push a ball into the first cluster and see what happens.

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In the middle of the action (picture 2). Notice how the last ball in cluster two is blurry -- it is currently in motion - going up the ramp.

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Aftermath (picture 3). What happened? It looks like we ended just where we started?

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Why? As the ball on the end of cluster 2 goes up the ramp, it is gaining potential energy as a result of gaining height. Once it reaches its apex, it will return down the ramp and impact with cluster 2. The imparted momentum is great enough to release the ball on the opposite end of the cluster resulting in picture 3.

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    78 Comments

    Could this be turned into some sort of perpetual motion machine? Doubtful seeing depressing new-age thermodynamic laws. What changes would need to be implemented to keep this going in a loop?

    Perpetual motion does not exist. Imagine a long track of the magnets/ball bearings arranged in a circular pattern. Consider an unfired pair of bearings as the ground state, then fire and you will see that the ground states activated by the processs of firing cannot fall back on their own to their original ground state where the loop is tied up. Undo this by adding more bearings? You might get lucky and have the thing run a few loops (might = no chance). You will always end up having an activated state (taken up energy, none to release), that has to give energy to activate something which is already in its activated state and that cannot fall back without energy input. Dead end. Just began typing this response, then scrolled down to see the rest... Sorry, could go on at length with the answer... The only (almost) perpetual machines around are atoms, but then again, they are not machines: their electrons keep spinning (rather, existing) around the nucleus for (an estimated) 1x10^30 years. That's (almost) perpetual (and useful) because it keeps my body intact and prevents me from becoming a black hole, which reminds me of a very nice Gary Larson cartoon...

    Perpetual motion does exist, just not in any state achievable on earth. The formula should go something like... momentum+resistance >= gravity+friction

    what about the earth around the sun, or the moon around the earth? perpetual motion?

    Newton's Laws of motion explain fairly well... Earth is sufficiently far away and of significant mass that small forces are nearly insignificant on Earth'y orbit. Comets, however, make up for a lack of mass with velocity which is why they can have open orbits and escape the sun.

    The moon, on the other hand, is slowly being pushed away from Earth. About 4cm per year resulting in Earth's rotation slowing about 2 seconds every 100,000 years due to tidal forces and tidal locking (same reason we only see one side of the moon).

    Satellites experience significant orbital decay as their mass is much smaller compared to Earth's mass and their distance is EXTREMELY close. Small forces due to atmospheric particles have large consequences over time.

    The Earth, Sun and Moon are in motion, but their motion is not perpetual.

    we're way off topic about your rail gun, which is a cool display of physics by the way, and i'm sorry for this, lol. I get and agree with the moon thing, I'm not so conviced about the earth around the sun. newtons first law says, well you know what it says. there is no fluctuation or transfer of energy, just a perfect balance between gravity and centripetal force. is that not perpetual? lol, i'm really sorry, these things just eat at me until i get an answer i can understand. what do you think?

    Balanced forces does not necessarily mean perpetual. They just mean forces are balanced.

    The Earth happens to be travelling through a vacuum - there really isn't much that will slow it down but things do. Meteorite strikes, technically, are forces on Earth. Earth has several orders of magnitude more mass - this is like throwing a grain of sand at a cannon ball. Sure, the grain of sand applies a force, but it's imperceptible.   Other than meteors, nearby asteroids and other celestial bodies will exert nonzero gravitational forces - Earth is still much larger - so large that it's irrelevant here...

    To note: Earth is slowly moving away from the Sun - about 15cm per year. Slowly, we're stealing angular momentum from the Sun so that we may move into a higher orbit. On a human scale, this orbit is forever and perpetual. For the rate at which this occurs, the Earth will be swallowed by the Sun long before there's any orbit issue.

    cool. well thanks for indulging me. it's fun for me to learn about stuff like this and to be able to ask a few questions. the science channel doesn't do a good job of answering when i talk to it, lol.

    also if you had a "prepetual waterwheel machine" it would stop because of friction from the wheel turning

     not unless you use a portals gun and have the track on a ramp.  : )